1. Field of the Invention
The present invention relates to low temperature impact resistant, easily flowing, thermoplastic polyurethane elastomer compositions comprising a mixture of thermoplastic polyurethane elastomers (A) and (B) having different hardnesses, and optionally reinforcing fillers, and a process for their preparation and their use.
2. Description of Related Art
Thermoplastic polyurethane elastomers, henceforth abbreviated TPU, are well known. Their technical significance is based on the combination of highly useful mechanical properties and advantages of economic thermoplast processing. By using different chemical starting components in varying quantity ratios, products can be prepared which are very different with respect to their processibility and their mechanical properties. An overview of TPU, its properties, and applications is found, for example, in: Plastics 68, 1978, pages 819-825, or in The Plastics Handbook, Volume 7, Polyurethanes, Second Edition, edited by Dr. G. Oertel, Carl-Hanser Publisher, Munich, Vienna, 1983.
TPU can be prepared continuously or batchwise according to different processes. The most well known is the conveyor process, but also industrially useful is the extruder process.
GB-A-10 57 018 discloses preparing a prepolymer from an essentially linear polyhydroxyl compound and excess organic diisocyanate which is fed via a metering pump into a mix head and is mixed there with a certain quantity of a lower molecular weight diol. The resulting reaction mixture is applied to a conveyor belt and fed into an oven heated to from 70.degree.-130.degree. C. until it hardens. Then the reaction product is reduced in size, stored 6 to 40 hours at temperatures up to 120.degree. C., and in this form it can be processed, for example, into molded particles using injection molding machines. The post curing and size reduction steps are what increase costs for the conveyor process.
Following another process variation, a linear polyhydroxyl compound, a lower molecular weight diol and the organic diisocyanate can be fed into a mix head via separate lines, mixed there, and finally the reaction mixture obtained is applied onto a conveyor belt.
In an extruder process, for example in DE-A-20 59 570 (U.S. Pat. No. 3,642,964), the starting components are fed directly into the extruder and the reaction is carried out in the extruder under certain process conditions. The polyurethane elastomer formed is transferred into a thermoplastic state, extruded, cooled until hardened in an inert gas atmosphere, and reduced in size. A disadvantage to this process is that the TPU obtained, having Shore A hardnesses up to 95, is unsuitable for preparing films or delicate shapes and tubes. Moreover, such films often block and stick together so strongly that subsequent processing is impossible.
To overcome the aforesaid disadvantage, in EP-A-111 682 (CA 1 236 239) easily demoldable and non-blocking TPU is described which comprises 50 to 99.5 weight percent of a TPU (A) comprising 10 to 50 weight percent of urethane groups and having a melt index of 0.1 to 100 at 190.degree. C., and from 0.5 to 50 weight percent of a TPU (B) comprising 20 to 60 weight percent of urethane groups and having a melt index of 10 to 1000 at 190.degree. C., whereby the urethane group content of TPU (B) is always at least 10 weight percent greater than that of TPU (A), and the melt index of TPU (B) at a given melt temperature is equal to or greater than the melt index of TPU (A).
U.S. Pat. No. 3,284,539 discloses stable TPU molding compositions prepared by melting together a TPU having terminally positioned hydroxyl groups and a TPU having terminally positioned isocyanate groups. A disadvantage of these TPU molding compositions and the above mentioned non-blocking TPU's, particularly those having Shore A hardnesses greater than 95, is that they have an insufficient low temperature impact resistance and flowability is poor.
U.S. Pat. No. 4,129,611 discloses TPU molding compositions from rigid and flexible TPU which demonstrate insufficient solidification associated with insufficient low temperature impact resistance. This patent discloses flexible TPU prepared from organic polyisocyanates and higher molecular weight polyhydroxyl compounds or mixtures having average hydroxyl equivalent weights of at least 200 from higher molecular weight polyhydroxyl compounds and lower molecular weight chain extending agents; and rigid TPU prepared from organic polyisocyanates and compounds having reactive hydrogen atoms and an average hydroxyl equivalent less than 100, mixed in the melt or preferably in the form of a solution, for example, in dimethylformamide as a solvent.